Deficits in interneuron generation and function are thought to underlie the pathophysiology of epilepsy and cognitive disorders including schizophrenia, autism spectrum, and anxiety. To understand interneuron development and brain construction we will compare and contrast the actions of G1-phase cyclin D2 (cD2) versus cyclin D1 (cD1), cell cycle proteins that regulate passage through mid-G1 toward S-phase. These D- type cyclins are expressed in different progenitor pools of the embryonic mouse neocortex and the medial ganglionic eminence (MGE), the origin of cortical parvalbumin (PV)- and somatostatin (SST)-expressing interneurons. We previously showed that cD1 is highly enriched in radial glial cells in the ventricular zone, while cD2 predominates in intermediate progenitor cells in the subventricular zone (SVZ) of the neocortex and MGE. Cyclin D2 knockout mouse embryos exhibit small cortices, prolonged G1 duration, and premature cell cycle exit. Adult animals deficient in cD2 show disproportionate reductions in PV-positive interneurons with preserved SST interneuron densities, while cD1 knockout animals do not exhibit such deficits. Thus, cD2 expression is crucial for proliferation and production of specific subpopulations of interneurons. The mechanisms by which cD2 and cD1 differentially regulate neural proliferation are poorly understood. Recently, we identified human mutations in known phosphorylation sites on cD2 that result in severe cortical malformations, indicating the importance of phosphorylation of this molecule for brain construction. This fellowship application will explore the role that phosphorylation of cD2 versus cD1 plays in embryonic forebrain neurogenesis. Based on our previous work, we hypothesize that cD2 and cD1 are differentially utilized in embryonic brain development so that cD2 promotes SVZ symmetric cell divisions. We will employ in utero electroporation, ex vivo slice electroporation, and time-lapse imaging to examine the role of cD2 in proliferation and promotion of asymmetric versus symmetric cell divisions. We will also use pair-cell in vitro assays and in vivo genetic mosaic analysis with double markers (MADM) to assess asymmetric versus symmetric division outcomes in the presence or absence of cD2. The proposed experiments will advance our understanding of precisely how these D-type cyclins regulate proliferation within the cerebral cortex and the MGE, illuminating how interneuron subtypes are generated in the MGE. These studies will contribute to the ultimate goal to devise more effective therapies for neuropsychiatric disorders.